1. Field of the Invention
The present invention relates to a ferroelectric memory, for example, a ferroelectric memory of an RC-parallel-unit serial connection type.
2. Background Art
An RC-parallel-unit serial connection type is known as a type of an FeRAM (Ferroelectric Random Access Memory). The FeRAM of the RC-parallel-unit serial connection type includes serially connected structures in each of which a ferroelectric capacitor and a transistor are connected in parallel (the structures are called “RC parallel units”). The RC parallel units will be simply referred to as “units”, hereinafter.
In the FeRAM of the RC-parallel-unit serial connection type, two blocks, each of which includes plural units, often share a plate line.
It is now assumed that first and second blocks share a first plate line and respectively include N1 and N2 units, and that third and fourth blocks share a second plate line and respectively include N3 and N4 units. With regard to these blocks, it is assumed that the first and third blocks are located on the same side, and that the second and fourth blocks are located on the same side.
Conventionally, values of N1, N2, N3, and N4 are normally set to be equal for ease of design. Further, the value of N1, N2, N3, and N4 (N1=N2=N3=N4) is normally set in power series of 2. Recently, however, following the progress of high integration of the FeRAM, etching pitches of memory cells, bit lines, word lines, and plate lines are narrower. As a result, it is difficult to arrange plate lines generally made of Cu or Al to satisfy N1=N2. Therefore, cases where the plate lines are arranged to satisfy N1≠N3 have increased.
Accordingly, in the FeRAM of the RC-parallel-unit serial connection type, cases of mixing blocks different in the number of the units have increased. For example, the values of N1 to N4 are set to satisfy N1+N2=N3+N4=8, N1=3, N2=5, N3=5, and N4=3.
In such a case, a method for redundancy repair is a problem. To reduce a circuit area, redundant cells for the FeRAM are preferably provided only on either the first and third block side or the second and fourth block side. For example, if the redundant cells are provided only on the first and third block side, five redundant cells are connected to the first block. Further, three redundant cells are connected to the third blocks according to symmetry of a memory cell array. In this case, it is sufficient to use the former five redundant cells to repair the first and second blocks. However, the problem is that it is insufficient to use only the latter three redundant cells to repair the third and fourth blocks.
JP-A H9-148533 (KOKAI) describes an example of a semiconductor memory device having a function of repairing a defect in a memory cell array. In the semiconductor memory device described in the JP-A H9-148533, an access to a word line or a bit line including a defective part in a normal memory cell array is replaced by an access to a word line or a bit line in a redundant memory cell array.